An electrophotographic process typically comprises six main steps: charging, photocharging, imaging, image transfer, development, and cleaning. Toner developers are used in the image transfer step to transform electrostatic latent images into positive images, which are then transferred onto papers or transparent films to produce printed copies. With a binary- or two-component developing system, the toner developer contains toner particles and carrier particles. The toner particles are first electrostatically charged through triboelectrification with the carrier particles, so that the charged toner particles can be subsequently electrostatically attracted to an electrostatic latent image on a photoconductor. Electrophotographic processes using two-component toners have been widely used in electronic facsimile devices, mainly because of their excellent image resolution, and their high electric resistance which allows the image forming process relatively unaffected by humidity and other environmental variables.
Two-component toner systems, however, have exhibit several disadvantages in that they involve relatively complicated machine construction and are difficult to maintain. Furthermore, since a toner in a two-component toner system is triboelectrically charged by mutual friction between the toner and the carrier, the surface of the carrier will be contaminated with the toner after the two-component toner is used for a certain period of time. When this occurs, the surface of the carrier will be contaminated with a very thin layer of minute toner particles, thus making it difficult or even impossible to apply sufficient triboelectric charge to the toner.
In recent years, various electrophotographic processes utilizing a magnetic one-component toner developer have been developed. A one-component toner developer contains only the toner component and is free of any carrier. A magnetic one-component toner developer typically contains 30-60 wt % magnetic powders. U.S. Pat. Nos. 3,909,258 and 4,121,931 provide disclosures of such magnetic one-component toner developers, and the contents thereof are incorporated herein by reference. An electrophotographic process utilizing one-component toner developer has the advantages in that it involves a much simplified equipment construction, and is much easier to maintain. However, the magnetic one-component developers also have several disadvantages, for example: (1) because they contain large amounts (30-60 wt %) of magnetic powders, the electrical resistance thereof is substantially reduced, thus resulting in inferior resolution and susceptibility to environmental changes; (2) the large amounts of magnetic powders contained therein also adversely affect the thermal fixing ability of the resultant toner developers; and (3) because magnetic powders are mainly comprised of black colored Fe.sub.3 O.sub.4 filler material, this makes it essentially impossible to obtain non-black color toner developers.
In more recent years, electrophotographic processes utilizing non-magnetic one-component toner developer have also been developed. The non-magnetic one-component toner developer systems have a promising potential to provide all the advantages of the magnetic one-component toner developer systems, yet avoiding most if not all the of disadvantages thereof. However, it still remains a great challenge to develop a suitable non-magnetic one-component toner developer that fulfills it potential. First of all, a non-magnetic one-component toner developer must possess rapid triboelectrification characteristics as well as an ability to carry sufficient triboelectric charge, so as to allow the same to be adequately charged triboelectrically in order to form an image on a photoconductor during the very short duration of its frictional contact with a doctor blade or a conveyer roller.
Because of its inadequate triboelectric charging characteristics, a toner designed for a conventional two-component toner developer cannot be used in a one-component toner electrophotographic imaging process. However, in a two-component toner developer system this shortcoming is overcome by the additional blending and friction between the toner and the carrier to achieve adequate triboelectrification.
FIG. 1 is a simple illustrative diagram describing an electrophotographic device utilizing a non-magnetic one-component toner developer. Toner 2, which is stored in a toner storage tank 1, is carried by a spongy shaft carrier 3 to a developer sleeve 4. A doctor blade 5 controls the thickness of the toner on the developer sleeve. The frictional contact between the doctor blade 5 and the developer sleeve 4 causes the toner to become triboelectrically charged. The charged toner then is moved from the developer sleeve onto the electrostatic image on the photoconductor 6.
It has been recognized that, in order to ensure excellent printing quality using a non-magnetic one-component toner developer, it is very important to design and control the particle size of the toner developer and the distribution thereof. The particle size distribution of toners directly affects the their triboelectrification characteristics and flowability. If the toner has a particle size distribution that is too broad, it will not be able to provide good printing quality, as the broadly distributed toner particles are likely to contain excessive amounts of very large and/or very small particles. Both could adversely affect the triboelectrification characteristics and flowability of the toner composition. Very large particles not only cause the triboelectrification characteristics of the toners to be lowered, they also directly cause a deterioration of the resolution of the printed copies. Some of the problems observed from using toners that contain excessive amounts of larger particles include: protruded characters in the printed documents; blocky graphics which are rough and non-smooth; decreased shininess in the prints; increased toner consumption; etc.
If, however, the toner composition contains excessive amounts of very small particles, not only that the flowability of the toner will be impaired, thus causing non-uniform printing quality, the triboelectrification characteristics of the toner will also be adversely affected. The latter causes the problems of reduction in the print toner concentration, formation of ghost or fog background, deterioration in thermal fixability, as well as contamination of the copier or printer components. The main reason for these problems is that, due to their similarity in composition with the majority of the toner developer, these fine particles will possess excessively high triboelectricity, thus bringing in a competition with the main components of the toners during the image developing process. This results in a deterioration in the print quality. Furthermore, because of their excessively large surface area per unit weight, the fine particles will adversely affect the thermal fixing process, thus causing the printed characters or graphics to be easily peeled off. On the other hand, the fine particles, which are formed during the fine grinding step, often exhibit relatively non-uniform properties compared to the majority of the toner particles. This aberration causes some of these fine particles to be reversely charged or even carrying very low charges. The constitutes the main reason for the occurrence of ghost background.
From the above described considerations, it is hence extremely important to design and control the size distribution of the toner particles, in order to ensure consistent printing and/or copying qualities, i.e., a uniformly clear image in the solid region and no fog or ghost in the background region.
U.S. Pat. No. 5,063,133, issued to T. Kubo, et al., discloses a toner developer for electrophotographic process which satisfies the expression: d.sub.75 /d.sub.25 .ltoreq.d.sub.50 /40+1.2, wherein d.sub.25, d.sub.50, and d.sub.75 represent diameters of toner particles when the percentiles by volume or by weight or toner developer of that diameter are 25%, 50%, and 75%, respectively. The primary object of the '133 patent was to provide uniformly clear images without the fog or ghost background. However, the criterion disclosed in the '133 patent appears to be grossly inadequate, as illustrated below by results from tests conducted by the inventors. Many toner developers fit the criterion of the '133 patent but do not provide good printing quality. On the other hand, many toner developers that do not fit the criterion of the '133 patent actually provide excellent printing qualities. Therefore, a different but more rigorous as well as more accurate definition is necessary in order to design and control the optimum particle size distribution of toner developers.
In addition to the particle size distribution, other factors can be equally important in ensuring a good quality of the printed products. These factors include triboelectrification characteristics and flowability of the toner developer. European Pat. App. No. 0 438 245 A2 discloses a non-magnetic one-component toner developer comprising colored fine particles and inorganic powders, such as silicon dioxide, which have been surface-treated with a silicone oil, a silane coupling agent or a silazane compound, to impart hydrophobicity. The blending of the hydrophobically treated fine powders with the toner developer improves the triboelectrification characteristics and flowability of the final toner composition. One of the disadvantages of the toner developers disclosed in the '245 prior art is that the silicon dioxide particles, which exhibit the characteristics of primary particles before the hydrophobicity-imparting surface treatment, are likely to form primary aggregates or even bulk aggregates after the treatment, regardless of whether the treatment is done using a spraying method or a solution soaking method. The formation of the aggregates prevents the toner/silicon oxide composition from being effectively blended, and greatly undermines the benefits that were originally designed to improve the triboelectrification characteristics and flowability thereof.